For the best prices, book well ahead. Tickets go on sale 3 months before travel, and are cheapest then. See www.thalys.com
Train stations have departure screens, similar to those at airports. These show the departure time, the train number (on your ticket for all fast trains), the final destination (which may be beyond where you are going to), and the platform (voie) that will be used.
Kim Soon L: As others have noted, your departure time will be shown on the display boards, but....you might see your specific train listed, but the platform might not show up until 15 minutes before the train leaves. So, since it sounds like you are a bit anxious about it, here are some thoughts.
Unlike the airport, you can just show up 20-30 minutes before your train is scheduled to leave. Barring strikes, the train will leave at the time noted. Be ready when the Departures board shows your train platform. Once it appears, go there immediately. I've forgotten if Nord has the "train coach plan" shown on the platform, but be sure to look for it. It will show you almost exactly where your specific coach will stop on the platform when the train pulls into the the station. It will be a great help for you to be standing close to where you need to get on.
You should know your coach and your SEAT(s) numbers before you board the train, you don't want to be uncertain and try and check your tickets as you board. Compare your ticket listing to the coach number on the outside of train before you get on. Especially with four people, you don't want to get on the wrong coach and then walk through the coaches to find yours. Then be certain you are in your correct seats....others travelers might not be pleased if you are in their seats.
Trains do not have baggage checking....sorry if you already know this,... so any luggage your family of four plans to get on the train, you will have to get on to the train yourself. Usually one (or two) steps up from the platform. So, pack as light as you can. Most trains have only limited "large size" luggage storage bins (usually just as you get on the train), so small luggage and hand bags will come into the train car with you.
All of this is going to happen with a bunch of others all trying to do the same thing at the same time....deep breathe, and don't get flustered.
(Have a great trip...Amsterdam is a favorite of mine.)
Bookings open 4 months in advance of travel date, and the route is very busy so the cheapest tickets go quickly. The journey takes around 3h15 and a bit longer on weekends.
The Thalys trains are easily identifiable as they are a dark red colour and usually leave from platforms 8-10 but check overhead screens to be sure. Your exact platform wont be displayed till around 15 mins before departure. Be wary of pickpockets at the station.